Chemical Mechanical Planarization (CMP), or Chemical Mechanical Polishing is a process commonly used to remove topography from wafers during the manufacturing of integrated circuits (ICs). Generally, CMP is a process of smoothing and planing surfaces with the combination of chemical and mechanical forces. CMP is most widely utilized in back-end IC manufacturing.
FIG. 1 shows a typical CMP apparatus and some of the primary components. A CMP head 10 is coupled to a CMP machine by spindle 13 and transmits movement (linear, rotary, and/or orbital) to a wafer 12. Head 10 also carries the appropriate connections to support the internal workings required by head 10 in the CMP process (e.g. pneumatic lines). Head 10 includes carrier 14, which supports a wafer backer 15 and a retaining ring 18. Backer 15 is designed to load/unload the wafer as well as provide physical support and process control options during processing (e.g. apply back pressure and allow wafer to float within a given tolerance during processing). Retaining ring 18 is removably coupled to carrier 14 and is designed to hold the wafer within the head 10 during CMP processing. Opposite carrier 10 is polishing pad 22. Polishing Pad 22 is coupled to a rotatable platen or table 26, which causes the polishing pad 22 to rotate as shown by rotation line 24.
During the CMP process, wafer 12, being removably coupled to a head 14, is inverted such that the integrated circuit-embodied surface opposably faces a polishing pad 22. Polishing pad 22 is saturated with a slurry 30 that may contain abrasive particles and a mild chemical etchant that softens or catalyzes the exposed surface of wafer 12 being planarized. Wafer 12 is polished by placing it into contact with the polishing pad 22 and slurry 30 while the polishing pad 22 is rotated. The surface roughness of the integrated circuit-embedded exposed surface of wafer 12 is removed by the combined action of chemical softening of the exposed surface of wafer 12 and physical abrasion brought about by relative movement and pressure between the polishing pad 22, the slurry 30 and wafer 12.
Because Retaining ring 18 keeps wafer 12 in position during the polishing process, the ring wear edge 28 also contacts the polishing pad 22 and slurry 30. Accordingly, retaining ring 18 may succumb to wear, and thus has a finite life. As retainer ring 18 wears, one or more portions of wear edge 28 may tend to recede or move “inward”. Because backer 15 is designed to float, as retaining ring 18 wears, the wafer 12 also floats such that the retaining ring will still be effective despite wear. When retaining ring 18 wears to a point such that it begins to float (i.e. the wafer and backer are past their designed mechanical tolerance), the wafer will no longer be held in place and may be undesirably ejected from carrier 10. At this point, one or more portions of wear edge 28 may be considered as having receded or moved “inward” along respective inward directions in excess of one or more failure thresholds. Another problem with too much wear, particularly on retaining rings that are slotted on the ring edge 28 to allow slurry and waste to enter and exit the retaining ring perimeter, is that the flow characteristics of the slurry will be changed which can negatively impact the planarization process.
The current industry practice for monitoring retaining ring wear and failure include manual inspection or estimation. Ring edge wear (i.e. inward recession of the wear edge) is often monitored by periodic inspection and measurement using instruments such as a caliper and the like. This technique, however, is time consuming, inaccurate and increases the potential for system contamination. Accordingly, users typically opt to routinely change out the retainer rings after processing a certain number of wafers. Based on factors such as the processing parameters, retaining ring material, slurry composition, pressure and the like, the typical change out is made long before the retaining ring is worn out. Though erring on the side of caution prevents damaging wafers and helps to ensure process consistency, replacing retaining rings with a substantial amount of life remaining wastes resources and money.